In modern textile mills, fabric or cloth is commonly tufted, woven or knitted in predetermined widths on high-speed, automated looms. It is common in such processes for a thread supplying the loom to break resulting in a flaw commonly known as an "out" in the fabric. Supply threads can also become entangled or misplaced in the fabric resulting in flaws known as "slubs". It is important that such flaws be detected soon after the cloth leaves the loom so that the loom can be stopped and the defect causing the flaw corrected.
In the past, such flaws have been detected through manual visual inspection by the operator of the loom. Such manual inspection has often proven to be inefficient and has become increasingly more difficult as the speed of modern looms has increased. In addition, manual detection of flaws in fabric having patterns or designs knitted therein is often even more difficult.
Automated optical scanning devices for detecting flaws in fabric as it leaves the loom have been developed. In general, these devices comprise means for illuminating one side of the fabric and means for detecting the intensity of light that is transmitted by the fabric. The detecting means has included moving or scanning type sensors such as those illustrated in U.S. Pat. Nos. 4,702,283 of Shaw and U.S. Pat. No. 3,410,643 of Jorgensen. These types of sensors tend to have many mechanical moving parts and are prone to require frequent adjustment and repair. In addition, such scanning detectors only inspect the fabric in zig-zag lines across the width of the fabric such that the entire surface of the fabric is not inspected.
Detectors comprising arrays or lines of optical sensors that inspect the entire width of the fabric as it leaves the loom have also been developed. U.S. Pat. No. 3,001,080 of Neil illustrates a device using such a sensor array.
It has been common, when using both scanning sensors and sensor arrays, to compare the intensity of light falling on each sensor to a predetermined value and, if the difference between the detected intensity and the predetermined value exceeds a preset difference, to produce a signal that stops the loom or activates an alarm so that the loom can be adjusted or repaired. It is common for the predetermined value and preset difference used for comparison to be preset manually by the operator as illustrated in the patent to Neil.
Prior art methods of detecting flaws in material, while representing improvements in the art, have not always proven acceptable. In particular, the prior art methods tend to perform poorly when detecting flaws in material having designs or patterns dyed or knitted into the material. This is because the designs themselves can cause extreme variations in the intensity of light transmitted by the material with such variations causing prior art methods to signal falsely a flaw in the material. This is particularly true for scanning type sensors that only inspect a fraction of the surface of the material.
In addition, even when used with fabric having no design, prior art devices tend to stop the loom upon a single detection of an unusually high or low intensity. This often causes the loom to be stopped for insignificant or acceptable flaws or because of an anomalous sensor reading resulting in costly time delays.
Accordingly, it is to the provision of a method and apparatus for detecting flaws in knitted fabric that overcomes the problems of the prior art that the present invention is primarily directed.